1. Field of the Invention
The present invention relates to how to make a layer-to-layer jump on a single-sided stacked multilayer optical disk with three or more information layers and also relates to an optical disk apparatus for performing read/write operation on such an optical disk.
2. Description of the Related Art
In optical disk technologies, data can be read out from a rotating optical disk by irradiating the disk with a relatively weak light beam with a constant intensity, and detecting the light that has been modulated by, and reflected from, the optical disk. On a read-only optical disk, information is already stored as pits that are arranged spirally during the manufacturing process of the optical disk. On the other hand, on a rewritable optical disk, a recording material film, from/on which data can be read and written optically, is deposited by evaporation process, for example, on the surface of a substrate on which tracks with spiral lands or grooves are arranged. In writing data on a rewritable optical disk, data is written there by irradiating the optical disk with a light beam, of which the optical power has been changed according to the data to be written, and locally changing the property of the recording material film.
In this description, an information layer on which data has been written and/or a layer on which data can be written will be referred to herein as an “information layer” or simply a “layer”. A multilayer optical disk is an optical disk in which a number of information layers are stacked one upon the other at predetermined intervals. In such a multilayer optical disk, the distance (or depth) from its light incident surface is different from one information layer to another. Also, when an optical disk is irradiated with a light beam that has been converged by an objective lens, a spherical aberration is produced. The magnitude of such a spherical aberration varies according to the depth of the target information layer but is preferably as small as possible. That is why in order to condense a light beam onto the target information layer, the condensing state of the light beam needs to be adjusted and the magnitude of the spherical aberration needs to be changed so that the spherical aberration is minimized on that information layer.
For a multilayer disk, a so-called “layer-to-layer jump” (which is also called a “focus jump”) technique, which is a technique for moving the target focus position of a focus actuator quickly from one layer to another, is an indispensable technique. However, after the layer-to-layer jump operation has been started, the focus may be set by mistake on a wrong information layer instead of the target layer.
Japanese Patent Application Laid-Open Publication No. 2009-230781 discloses a method for determining on which layer the focus position is now located after a layer-to-layer jump has been made on a double-layer BD. For example, before a jump is made from L0 layer to L1 layer, the spherical aberration is corrected with a spherical aberration correction value set for a collimator lens driver. After that, the layer-to-layer jump is made. If the jump to the L1 layer has been done successfully, then the TE signal should have the same amplitude before and after the jump. On the other hand, if the jump has failed and the focus has been set on the L0 layer again by mistake, then the amplitude of the TE signal after the jump has been made is much smaller than that of the TE signal before the jump is made. Thus, by comparing the amplitudes of the TE signal before and after the jump with special attention paid to that characteristic, it can be determined whether or not the focus position has reached the target layer successfully.
Meanwhile, Japanese Patent Application Laid-Open Publication No. 2007-095218 does not disclose a layer-to-layer jump method but a method for determining on which layer of a multilayer disk the focus position is currently located during a focus search operation. For that purpose, the amplitude of a TE signal (or an RF signal or any other similar signal) associated with the target layer is stored in advance. And the amplitude of a TE signal (or an RF signal or any other similar signal) that has been measured on the layer on which the focus has been set is compared to the amplitude stored, thereby determining whether or not the focus position has reached the target layer.
That is why with the technique disclosed in Japanese Patent Application Laid-Open Publication No. 2009-230781 or No. 2007-095218 adopted, it can also be determined, even in a multilayer disk (which is typically a multilayer BD), whether the layer on which the focus position that has been shifted through either a focus search operation or a layer-to-layer jump operation is currently located is a target layer or not.
However, the techniques disclosed in both of those two patent applications are basically designed for a double-layer BD. That is why unless the layer on which the focus position is currently located turns out to be the target layer as a result of the layer decision process, the layer-to-layer jump operation should be retried again so that the focus position can reach the target layer. Or a recovery process should be performed by defocusing the light beam once and then attempting to set the focus on the target layer all over again.
Suppose in a triple-layer disk in which L0, L1 and L2 layers are stacked one upon the other, the focus position needs to be shifted from the L0 layer to the L1 layer through a focus jump operation. In that case, if the amplitude of the TE signal measured on the current layer is smaller than either a predetermined value or that of the TE signal associated with the L0 layer, then it can at least be seen, even according to the conventional techniques, that the current layer is not the L1 layer. However, according to such methods, it cannot be determined whether i) the focus position has gone over the L1 layer to reach the L2 layer or ii) has come back to the L1 layer by mistake.
Consequently, in the prior art, once a layer-to-layer jump has failed on a multilayer disk, it is not clear in which direction the focus position needs to go, no matter whether it should attempt to reach the target layer once again or just go back to the original layer. That is to say, in such a situation, it is so difficult for the focus position to go directly to the target L1 layer or simply go back to the original L0 layer that the system might sometimes fail or it would take a lot of time for the system to make a recovery.
It is therefore an object of the present invention to provide an optical disk apparatus, which can always shift the focus position from one of three or more layers of a multilayer disk to another both quickly and with good stability by re-setting the focus position swiftly even if a layer-to-layer jump operation has once failed on any of those layers and if the focus has been set on a non-target layer by mistake. Another object of the present invention is to provide a method for making a layer-to-layer jump on a multilayer optical disk using such an optical disk apparatus.